The invention relates to a method for producing porous structures in which a liquid or pasty mixture of substances is brought at least partially to solidification and subsequently freeze-dried.
Sponges for cosmetic and medical applications can be manufactured by freezing an aqueous basic suspension by lowering its temperature and subsequently freeze-drying the frozen material. This method typically involves filling the basic suspension into metal bowls, which are subsequently cooled down from underneath using a cold brine, so that the basic suspension solidifies. This involves the suspension first solidifying at the bottom of the bowl and then also in the entire bowl. As a consequence of the expansion of the ice, a solidifying structure is provided, that after the process produces a very irregularly porous sponge with variously distributed density and various different pore orientations.
Another known method involves freeze-drying products in small bottles. This method involves the generation of a very varied solidifying structure in radial directions inside the small bottle. Supercooling in the outer areas of the bottle leads to the formation of globule-like ice crystals, while the solidification process is dendritic or cellular in the inner area of the bottle.
These differentiated degrees of porosity inside the product obstruct freeze-drying, as less porous areas of the structure generate strong material transport resistances. Moreover, products with homogeneous porosity are of great importance for many technical and medical applications.
It is an object of the invention to provide a process for producing products with a more homogeneous structure than the above-described processes.
In one aspect, the invention relates to a method for producing porous structures, in which a liquid or pasty mixture of substances is brought at least partially to solidification and subsequently freeze-dried The mixture is preferably cooled down between two surfaces with different temperatures that can be tempered and are set at a distance from each other which results in the generation of a substantially ordered or homogeneous structure during the solidification process.
In one aspect of the invention, the difference between the temperatures of the opposing surface areas is kept substantially constant during the cooling process. In another aspect, the temperature of one surface area is kept constant until the other surface area has cooled down sufficiently for the desired gradient to be created. The surfaces may be arrayed parallel to each other or are arrayed concentrically.
In another aspect of the invention, the mixture is introduced on a first side between the surfaces and on a second side is let out between the surfaces, while the temperature of the opposing surface areas drops from the first side to the second side. The mixture may, in a particular aspect of the invention, be fed through or along heat-conducting conveyor belts or hoses.
In a preferred aspect of the invention, the at least partially solidified product is freeze-dried, as one surface is removed and the other surface is tempered.
The liquid or pasty mixture may comprise or may consist of an aqueous solution or suspension. The liquid or pasty mixture may comprise an active ingredient, e.g., a biologically active material, living cells and/or a cryoprotective substance. Such materials (e.g., active ingredient, biologically active material, living cells and/or cryoprotective substance) may be added directly to the liquid or pasty mixture and may additionally (or alternatively) be added directly to the freeze-dried structure.
In another aspect, the invention relates to a method for producing porous structures in which a liquid or pasty mixture of substances is brought at least partially to solidification and subsequently freeze-dried, wherein the mixture is pre-cooled under pressure and subsequently suddenly depressurized. In a preferred embodiment, the pressure is in the range from 1 to 250 MPa. Additional gases may also be dissolved in the mixture.
In a related aspect, after cooling has been completed, the mixture is warmed back up to and temporarily maintained at a re-crystallization temperature before being freeze-dried.
In another aspect, foreign materials in preferred geometries and orientations are introduced into the liquid or pasty mixture and removed after the freeze-drying.
The methods of the invention are suitably employed in the manufacture of medicinal or pharmaceutical products or products of environmental and biotechnology, and are also suitable for producing sponges for cosmetic or medicinal purposes.